The invention relates to anaerobic treatment of low to high strength industrial and municipal wastewaters and for polishing the effluent from new and existing anaerobic systems.
Wastewater is water used indoors including water that drains from sinks, showers, toilets, washing machines, dishwashers, and any activity that uses water in some way in homes and businesses. Wastewater is typically classified in one of three categories: high strength, medium strength, and low strength. These classifications are usually based on several factors, such as the amount of organic material in the water, measured as biochemical oxygen demand (BOD), the amount of solids xe2x80x9cfloatingxe2x80x9d in the sewage, measured as total suspended solids (TSS) or suspended solids (SS), the amount of dissolved oxygen in the wastewater, the acidity/basicity of the wastewater, and the temperature of the wastewater.
Bacterial metabolism that occurs in the absence of oxygen is called anaerobic. Anaerobic microorganisms have played a major role in traditional municipal wastewater treatment through anaerobic digestion, which has been used to degrade organic solids and stabilize waste sludges from activated sludge processes. Over the past several years, anaerobic methods have been increasingly used for industrial pretreatment to remove suspended and soluble organic matter from aqueous streams, especially in high-strength process waters.
Waste waters from chemical, pharmaceutical, pulp and paper, food, dairy, brewery and meatpacking industries are being treated successfully today with a variety of anaerobic treatment systems. Anaerobic processes are normally operated at elevated temperatures (85-95xc2x0 F.) in the pH range of 6.8 to 7.4, and convert soluble organic carbon into carbon dioxide and methane, in contrast to aerobic systems which only produce carbon dioxide. The methane by-product of anaerobic systems is often used as a fuel to supply heat to the reactor.
Significant disadvantages of aerobic wastewater treatment processes over anaerobic processes are that aerobic processes require large amounts of oxygen and larger volumes for oxygen transfer, making the systems less cost effective. With high temperatures, or a combination of temperatures, anaerobic digestion can produce a high quality effluent, as in the TPAD process (Han et al., 19971) or anaerobic filters operated at thermophilic and mesophilic temperatures (Harris and Dague, 19932).
The use of anaerobic treatment for high-strength wastewater (generally where the chemical oxygen demand (COD) is above 3,000-4,000 mg/l or biological oxygen demand (BOD) is above 1,500 mg/l) eliminates the process limitations and problems associated with high oxygen demand and excessive biomass production that characterize traditional aerobic treatment systems. Another significant advantage of the anaerobic process for treating wastewater is that the anaerobic bacteria may release enzymes that help solubilize the organic solids in the influent, which means that both soluble and suspended BOD can be treated anaerobically.
A significant drawback of anaerobic wastewater treatment systems is that they typically require complex operation and control equipment. For instance, anaerobic systems often require mixing devices, gas or feed recirculation lines, and/or liquids/solids separators. Although necessary, these additional components often lead to operating problems.
Another disadvantage is that effluent from anaerobic processes usually requires costly post-treatment, typically at a municipal treatment facility. In addition to upkeep of the on-site treatment facilities, there is usually a charge for municipal treatment based on BOD, TSS and nutrients (such as phosphorous and nitrogen).
Anaerobic systems are well suited to the treatment of slaughterhouse wastewater. They achieve a high degree of BOD removal at a significantly lower cost than comparable aerobic systems and generate a smaller quantity of highly stabilized, and more easily dewatered, sludge. Furthermore, the methane-rich gas generated can be captured for use as a fuel.
In most countries, anaerobic ponds have been used to achieve a high reduction in BOD, oil, and grease and suspended solids concentrations from the primary-treated slaughterhouse wastewater prior, to subsequent aerobic treatment. Unfortunately, the propensity for odor generation from anaerobic ponds has threatened their continued use in many areas. Consequently, new developments in anaerobic technology during the last several years have been of considerable interest.
The testing of high-rate anaerobic systems has been one of the active areas of research concerning slaughterhouse-waste treatment during the last decade. The 1970s saw the use of low-rate anaerobic digesters to treat slaughterhouse wastewater. These processes were essentially mixed digesters with a BOD loading of between 0.2-4 kg/m3-day, and have proven uneconomic due to their required size. Since this time, a variety of new high-rate anaerobic technologies have been developed to replace the anaerobic pond. Typically, these are characterized as having higher BOD or COD loadings (typically 5-40 kg COD/m3-day) than low-rate systems or anaerobic ponds. This permits a hydraulic retention time in the order of hours, rather than days. The gas generated by the anaerobic activity is methane-rich, but in most cases H2S is also generated at concentrations from 0.2-0.7% from slaughterhouse wastewater and may need removal.
The NewBio reactor (U.S. Pat. No. 5,616,304) is a downflow, intermittently mixed, sludge blanket anaerobic reactor. This system consists of a covered circular tank and a water transfer/gas handling skid. The reactor includes upper and lower influent mixing blades, sand bed, fluidizing blade, slotted effluent drain, and a methane gas containment/storage area. The effluent from this system, however, requires post-treatment by filtration through a sand bed. This sand filter further has the drawback of causing clogging in the system.
An ideal biological treatment process would be easy to operate and produce a high quality effluent in a relatively small reactor volume. To achieve a high degree of organic removal at short HRTs, many anaerobic processes take advantage of anaerobic bacteria""s ability to form a dense agglomeration of particles called granules. Under anaerobic conditions in the reactor, organics from the wastewater are used by different types of microorganisms as the source of energy for the biological degradation process. These organisms tend to agglomerate into flocs, referred to as sludge. Under certain circumstances, the bacteria form small roundish pellets, called granules that consist mainly of methanogenic bacteria. The sludge produces gas as a by-product of the degradation process. A small amount of the food is transformed into either free energy and water or cellular material, which is equal to the new growth of bacteria. However, a large amount of the food is transferred into gas, which consists mainly of valuable methane and carbon dioxide. The Anaerobic Sequencing Batch Reactor (ASBR), Upflow Anaerobic Sludge Blanket (UASB), Anaerobic Migrating Blanket Reactor (AMBR), and other systems produce microbial granules during normal operation.
The formation of granules has been observed in many studies. Hulshoff Pol et al. (19833) found that most granules need an inert support structure to form upon, giving the organisms a building block. Others have noted that organisms adhere to other organisms forming the structure base. Usually, additional pressure is needed to force the organisms together, such as the velocity force from an upflow reactor. However, there is a selective mechanism which determines which groups will stay and which will be washed out in the process.
Granule formation is not limited to anaerobic organisms. However, most research has focused on anaerobic granules. The internal structure of the granule may vary depending on the type of substrate being degraded. Some granules contain layers created by different species of organisms working in a symbiotic relationship. Non-carbohydrate feed sources have been shown to produce homogenous, non-layered granules. Sucrose, brewery, and other wastewaters produce a visibly organized and layered structure because of the methanogenic conversion steps involved. In most anaerobic granules, the outer layer usually breaks down complex substrates into volatile fatty acids, which are then broken down to acetate and methane deeper into the granule. A wide consortia of organisms can be found on the surface of the granule. It has also been observed that filametous Methanothrix may be critical in the critical in the granulation process, possibly to hold the organisms together. Because of the array of organisms in close proximity and their interaction with each other, granules seem to have a high conversion rate and are easily adaptable to different substrates.
Several studies have shown the diverse microbial communities within the anaerobic granules. Because of this diversity, the granules are suitable for low strength wastewater, requiring shorter acclimation periods.
Several studies have been performed on low strength wastewater. Orozco (19964) achieved an optimum of 92% COD removal using an anaerobic plug flow reactor with an 11 hour HRT and operated at 13 to 17xc2x0 C. with synthetic wastewater. Ndon and Dague (19975) examined the performance of an anaerobic sequencing batch reactor (ASBR) at different HRTs and operating temperatures. With a substrate concentration of 1000 mg COD/L and HRTs of 24 and 12 hours, the soluble COD removal was 93 and 81%, respectively, while operating at 35xc2x0 C. Dague et al. (19986) used an ASBR to treat synthetic wastewater at low temperatures. The ASBR attained 90% COD removal or better for a feed strength of 600 mg COD/L at 20xc2x0 C. at HRTs greater than 8 hours. Collins et al. (19987) treated primary clarifier effluent with an expanded granular bed reactor (EGBR) and achieved greater than 90% COD removal at 20xc2x0 C. This system also had very low VFA concentrations.
The present inventors have now discovered an innovative new anaerobic process for treating low to high strength wastewater. The system is simple to operate and, in contrast to currently available anaerobic systems, does not require the use of mixers or support materials. The effluent from this novel system is low in COD, suspended solids, and volatile acids, which may allow it to be discharged to surface water without additional treatment.
It is therefore a primary objective of the present invention to provide an improved method of treating wastewater that achieves a high conversion of organics and pollutants.
It is another objective of the present invention to provide an improved method of treating wastewater that achieves high microbial biomass density, resulting in low concentrations of organic constituents leaving the reactor.
It is a further objective of the present invention to provide an improved method of treating wastewater that achieves high conversion of organics and pollutants and a faster rate than currently available methods.
It is still a further objective of the present invention to provide an improved method of treating wastewater that requires minimal equipment.
It is yet a further objective of the present invention to provide an improved method of treating wastewater that does not require mixers or solids/liquid separators.
It is a further objective of the present invention to provide an improved method of treating wastewater that does not require support material.
It is a further objective of the present invention to provide an improved method of treating wastewater that may be conducted at ambient temperatures.
It is a further objective of the present invention to provide an improved method of treating wastewater that utilizes a smaller constructed volume.
It is still a further objective of the present invention to provide an improved method of treating wastewater that is more economical to operate than previous methods.
It is still a further objective of the present invention to provide an improved method of treating wastewater that requires little upkeep for operation.
These and other objectives will become apparent from the following description.
The present invention relates to a method of treating low to high strength wastewater using a static granular bed reactor (SGBR). The SGBR is a high rate anaerobic treatment system that is capable of maintaining high solids retention times despite changes in hydraulic retention times. In addition, the SGBR can achieve a high level of organic removal and produce valuable methane gas from the wastewater.
The SGBR includes a fixed bed of anaerobic granules in a downflow configuration without flow recirculation. The resulting high granule density optimizes the contact between the microorganisms on the granule surface and the wastewater, which achieves a high conversion of organics and pollutants in a shorter time period. This results in smaller constructed volume, lower costs, and greater production of renewable energy in the form of biogas.
The configuration of the SGBR is simple, yet extremely effective. The only mechanical equipment required for its operation is an apparatus for feeding the wastewater into the reactor, such as a feed pump, and an optional auxiliary pump for occasional backwashing using the produced biogas. The SGBR uses a downflow bioreactor that is filled with active anaerobic granular biomass. Influent wastewater is distributed evenly across the bioreactor and passes downward through the granules. The gas that is produced by the granules provides channelization of the bed to prevent clogging. Clogging may also be prevented by recirculation of the gas or effluent to dislodge any trapped granules.
In contrast to previous wastewater treatment systems, the SGBR is simple to use since no extra equipment, such as mixers, sophisticated gas/solids/liquid separators, and heat exchangers are required. Its downflow configuration conserves biomass since it prevents washing out of the biomass. In addition, the SGBR does not require recycle pumping. Effluent from the SGBR is low in COD, suspended solids, and volatile acids concentrations, thereby eliminating the need for further treatment before surface discharge. The effluent may potentially be used as a non-potable water source for many industrial uses.